FIELD VALIDATION OF WATER-USE EFFICIENCY OF THE CAM PLANT OPUNTIA-ELLISIANA IN SOUTH TEXAS

In single plant studies, at time scales less than a growing season, th e conversion efficiency of water to dry matter several times has been shown to be greater: for CAM plants than either C-3 or C-4 plants. The objective of this work was to determine if this high conversion effic iency would occur at the held level over an entire growing season. Thu s, Opuntia ellisiana water-use efficiency values were obtained under f ield conditions in the third and fourth year of growth of a plantation . Although O. ellisiana is the slowest growing of all the spineless Op untias, it was used because it is the only spineless Opuntia that is c ompletely cold hardy in Texas. Above-ground dry matter production was obtained by complete harvest and by estimating above-ground biomass wi th regression techniques. Evapo-transpiration was estimated as the pre cipitation minus runoff, deep drainage, change in soil water content a nd water stored in the plant. In the fourth year, surface runoff was c aptured and measured in below-ground tanks. Based on the measurements, surface runoff in the third year was estimated using the regression t echnique. Soil evaporation was directly measured over a 2-week period, four times per year using microlysimeters. Using these direct soil ev aporation measurements and companion meteorological data, yearly trans piration was computed using a Radial Basis Function (RBF) neural netwo rk. This yearly soil evaporation estimate allowed the separation of tr anspiration from evapo-transpiration. At the beginning of the fourth y ears growth Opuntia ellisiana achieved a leaf area index (LAI) of 2.02 . In the fourth year, 662 mm rainfall occurred. Soil evaporation (214 mm) and runoff (143 mm) were the major losses. A significant quantity of water (17 mm = 170,000 kg ha(-1)) was stored in the cactus that cou ld be of use for animal drinking water. The cactus transpired 285 mm o f water producing 17,670 kg dry matter ha(-1) for a transpiration wate r-use efficiency of 162 kg H2O kg(-1) dry matter. This is among the gr eatest water-use efficiency of any plant species (including C-3 and C- 4) that has been measured under long-term held conditions. While this species is not particularly useful, other less cold hardy opuntias are widely used for fruit, vegetable and forage. Given water shortages on arid lands, much more development attention needs to be given to usef ul Opuntia species. (C) 1997 Academic Press Limited.